Just prior to beginning production, oil and natural gas wells are completed using a complex process called “fracturing.” This process involves securing the steel casing pipe in place in the wellbore with cement. The steel and cement barrier is then perforated with shaped explosive charges and the surrounding oil or gas reservoir is stimulated or “fractured” in order to start the flow of gas and oil into the well casing and up to the well head. This fracturing process can be repeated several times in a given well depending on various environmental factors of the well, such as the depth of the well, size and active levels in the reservoir, reservoir pressure, and the like. Because of these factors, some wells may be fractured at only a few elevations or depth locations along the wellbore and others may be fractured at as many as thirty (30) or more elevations.
As the well is prepared for fracturing at each desired elevational level or zone of the well, a temporary well completion plug is set in the bore of the steel well casing pipe with a setting tool just below the level where the fracturing will perforate the steel and cement barrier. When the barrier is perforated, “frac fluids” and sand are pumped down to the perforations, and into the reservoir. At least a portion of the fluids and sand are then drawn back out of the reservoir in order to stimulate movement of the gas or oil at the perforation level. Use of the temporary plug prevents contaminating the already fractured levels below.
This process is repeated several times, as the “frac” operation moves up the wellbore or “downhole” until all the desired levels have been stimulated. At each level, the temporary completion plugs are usually left in place, so that they can all be drilled out at the end of the process, in a single, but often time-consuming drilling operation. One reason the drilling operation has been time intensive is that the temporary plugs have been made of cast iron which has generally required several passes of the drilling fixture to completely drill out the plug. To reduce the drill out time, another type of downhole plug has been developed that is made of a composite material. Composite plugs are usually made of, or partially made of, a fiber and resin mixture, such as fiberglass and high performance plastics. Due to the nature of the composite material, composite plugs can be easily and quickly drilled out of a wellbore in a single pass drilling operation.
Temporary well completion plugs used in the fracturing operation described above, whether made of cast iron or composite materials, often come in two varieties, bridge plugs and frac plugs. Bridge plugs restrict fluid movement in either the upward or downward direction. Bridge plugs are used to temporarily or permanently seal off a level of the wellbore. Frac plugs generally behave as one-way valves that restrict fluid movement down the wellbore, but allow fluid movement up the wellbore.
In use, when frac fluids and sand are pumped down to a newly perforated level of the wellbore, a frac plug set in the wellbore just below the perforation level can restrict the frac fluids and sand from traveling farther down the wellbore and contaminating lower fractured levels. However, when the frac fluid and sand mixture is pumped back up the well to stimulate the reservoir at the newly fractured level, the one-way valve of the frac plug can open and allow gas and oil from lower levels to be pumped to the well head. This is advantageous to the well owner because it provides immediate revenue even while the well is still being completed.
Other situations exist, particularly during the completion of horizontal gas and oil wells, where the orientation of the wellbore precludes using gravity to lower the well plug into position. Instead, the plug is pumped into position by pumping fluid into the well from the surface, which process also requires that a portion of the well fluids flow to move the plug into position. If the plug or setting tool malfunctions, the operation of either a bridge plug or frac plug can prevent the placement of a second well plug by stopping the downward flow of fluid. Consequently, in horizontal wells each malfunctioning bridge or frac plug must be drilled out and replaced, again resulting in a time consuming and expensive operation.
Additionally, it will be appreciated that well completion plugs are typically fabricated as either a bridge plug or a frac plug, and one can not be converted into the other at the well site. Because of this limitation, well completion workers are forced to guess how many of each type of plug they will need to take with them to a remote well site so that they have plenty of both types of plugs for whatever the well conditions require. Unfortunately, this leads to expensive inefficiencies because either too many of one or both kinds of costly plugs are taken, or not enough of one or the other is taken to the remote well location.